In the wild, animals like whales, dolphins and some birds sleep with one hemisphere alert, but it was always thought that in humans, both sides shut down when resting.

Now, new research has shown that human brains unconsciously keep on side more alert when in unfamiliar surroundings. While it might seem a frustrating adaptation in the safety of a hotel room, in the past, it was probably an important evolutionary adaption.

"You don't sleep very well in a new place. We all know about it." said Dr Yuka Sasaki of Brown University.

"In Japan they say, 'if you change your pillow, you can't sleep. Our brains may have a miniature system of what whales and dolphins have.”

Researchers have long recognized that people sleep poorly the first night in a new location.

As a result, sleep scientists typically throw out data from the first night a person sleeps in the lab, only using data from the second sleep session on.

The left hemisphere of the brain stays awake when in strange surroundings Credit:
Alamy

To find out what was happening on the first night, scientists used advanced neuroimaging techniques to analyse the sleeping brain.

Over the course of three experiments their team used several methods to precisely measure brain activity during two nights of slumber, a week apart, among a total of 35 volunteers.

They consistently found that on the first night in the lab, a particular network in the left hemisphere remained more active than in the right hemisphere, specifically during a deep sleep phase known as "slow-wave" sleep.